US 4323833 A
A braking and speed-control circuit is disclosed which prevents excessive current from being dissipated in a DC series motor. The circuit includes two free-wheeling diodes, a forward-biased diode, and a thyristor. The forward-biased diode is placed between the field coil and the armature winding of the motor. One free-wheeling diode is placed across the entire motor circuit, including the forward-biased diode. The other free-wheeling diode is placed across the forward-biased diode and the field coil. The thyristor can short across the forward-biased diode and the armature winding.
1. A braking and speed control circuit for use with a DC series motor having a field coil and an armature winding which are entirely in series with each other, which motor is driven by a phase-controlled AC source, comprising;
a forward-biased diode intermediate the field coil and the armature winding and in series therewith;
a first reverse-biased diode placed in parallel with a first series network including only the field coil and the forward biased diode;
a second reversed-biased diode placed in parallel with a second series network including only the field coil, the forward-biased diode and the armature winding; and
a forward-biased semiconductor switch placed in parallel with a third series network including only the forward biased diode and the armature winding.
2. The circuit defined by claim 1, wherein the semiconductor switch is a thyristor.
3. The circuit defined by claim 1, further including a means coordinating operation of the semiconductor switch with phase-control of the source in a manner that by a time when the motion has been braked to a stop, the semiconductor switch is opened and the AC source is turned off.
4. The circuit defined by claim 3, wherein the semiconductor switch is a thyristor which is held conductive by electric current generated by the motor during braking.
5. The circuit defined by claim 3, wherein the means furthermore controls place of the AC source in dependance upon motor speed in order to hold motor speed to a constant value.
6. The circuit defined by claim 5, wherein the constant value is user-variable.
The invention relates to a control for a drive motor in a sewing machine. More particularly, this invention relates to a control system which will regulate motor speed in a stable fashion as such speed is increased and decreased and which will also stop the motor.
A prior art system of this kind is shown in FIG. 1, in which together with thyristors 2, diodes 3, form a full-wave rectifying bridge which supplies a DC motor. Thyristors 2 are phase-controlled by a gate control device (not shown) connected to their gates G2, and the motor 4 is thus speed-controlled by the phase-controlled voltages applied to gates G2, and numerals 7 and 8 are free wheel diodes. A braking thyristor 9 is connected in parallel with the armature 5, and after receiving a control signal (not shown) at its gate G9, short-circuits an armature 5 during motor braking, so as to consume the motor's rotational energy by dissapating it in the armature's own resistance. FIG. 2 shows the operating relations of the circuit elements during the drive and braking periods of the motor. In FIG. 2 line G2 designates a presence or non-presence of a gate signal of motor control thyristors 2, and during the period t1 to t2 indicates a phase-control voltage applied to thyristors 2. Line G9 shows a gate signal applied to braking thyristor 9, and the period between t2 and t4 shows a period of time in which the thyristor 9 is conductive (as shown by the level of line 9) to brake motor 4. Line N shows the speed of the motor 4, which during the period between t1 and t2 is driven at a speed determined by the gate signal G2. When thyristor 9 is conductive at the point t2 for braking the motor, field coil 6 is energized via short-circuited thyristor 9, since the thyristor 2 has been fired. Armature 5, therefore, generates reverse EMF from time t2 and the armature 5 carries current via thyristor 9. The rotational energy of the armature and the elements connected thereto is then consumed by the internal resistance of armature 5 and motor 4 is rapidly stopped. In FIG. 2, motor speed N is reduced at time t2, following the solid curved line. But, depending upon AC power source phase, speed, and motor load at time t2, the required stopping time for the motor can vary as is shown by dotted lines a and b. Therefore, in order to exactly stop the motor in a predetermined period, gate signals G2 and G.sub. 9 are prolonged respectively, to times t3 and t4. Accordingly, the field coil 6 is unnecessarily heated by a comparatively large electric current until time t3 --i.e. even after the motor stops. The prolongation of gate signal G9 serves the purpose of preventing the motor from rerotating. Such rerotation otherwise take place because cutoff of thyristor 2 is unstably dependent upon the phase of the AC power source at time t3, and may lag about a half cycle behind time t3, causing field coil 6 to be energized after cutoff of thyristor 9. This is the reason why such a timing control is necessary. However, such a timing control may cause erroneous operations of the circuit elements.
The present invention has been provided to eliminate such defects and disadvantages of the prior art.
It is a basic object of the invention to compensate for variations of the motor load and the power source over the whole speed range of the motor to thereby provide special stabilization.
It is another object of the invention to avoid supplying electric current to the motor while simultaneously generating electricity therein, to thereby stop the motor quickly and accurately.
Many other features and advantages of the invention will be apparent from the following description of its actual embodiments.
FIG. 1 is a prior-art control circuit;
FIG. 2 are graphs associated with FIG. 1;
FIG. 3 is a schematic diagram of the basic parts of invention;
FIG. 4 are graphs associated with FIG. 3;
FIGS. 5A and 5B represent circuit showing an actual embodiment of the invention, including the circuit of FIG. 3;
FIG. 6 are graphs associated with FIGS. 5A and 5B;
FIG. 7 is a motor speed detecting circuit used in combination with the circuit in FIGS. 5A and 5B;
FIG. 8 is a schematic perspective drawing of a sewing machine embodying the invention;
FIG. 9 are graphs associated with the circuit in FIG. 7;
FIGS. 10 to 12 are further graphs associated with the circuit in FIGS. 5A and 5B;
FIG. 13 is a second embodiment of the invention;
FIG. 14 are graphs associated with the embodiment in FIG. 13; and
FIG. 15 are further graphs associated with the embodiment of the invention shown in FIG. 13.
In FIG. 3, thyristors 10 and diodes 3 constitute a full wave bridge rectifier. G10 designates gate terminals of thyristors 10, to which gates a motor control signal can be applied. Reference numeral 11 indicates a DC motor having an armature 5 to which a field coil 6 is connected via diode 12 which conducts current in the same direction do thyristors 10. Braking thyristor 13 conducts current in the same direction as do thyristors 10 and is connected in parallel with a series network of armature 5 and diode 12. G13 designates a gate terminal of thyristor 13. A second braking diode 14 conducts in an opposite direction to thyristors 10 and is connected in parallel with a series network of field coil 6 and diode 12 to serve as a freewheel diode for field coil 6.
FIG. 4 differs FIG. 2 in that braking thyristor 13 receives a gate pulse G13 at time t5 having a width which is considerably narrower than the braking period. Just after thyristor 13 is fired the motor is braked and thyristor 13 cuts off when motor speed N is reduced to zero at time t7. However, depending upon loading conditions of motor 11, the conductive period of the thyristor 13 is extended (as shown by the dotted line) until the speed N of motor 11 ceases at time t'7. This will be explained with reference to FIG. 3. While thyristors 10 are ignited by gate signal G10 and motor 11 is rotated, braking signal G13 and maximum signal G'10 are delivered to braking thyristor 13 and thyristors 10 at time t5, for braking motor 11. In FIG. 4, signal G'10 is shown at a level higher than signal G10 for the sake of convenience to show that thyristors 10 are fired for the maximum ignition angle. Such an ignition angle temporarily increases current flowing through field coil 6 to make the motor brake more effective. Braking signal G13 also fires thyristor 13 for a maximum ignition angle for the same purpose. Field coil 6 is thus strongly energized by the maximum ignition of thyristors 10 and 13, and, on the other hand, armature 5 is not supplied with current because it is short-circuited by thyristor 13. When each of the ignition signals disappears at time t6, thyristor 10 is cut off at a zero crossover point of AC power source 1. Meanwhile, thyristor 13 continues to discharge the electromagnetic energy stored in field coil 6 and simultaneously consumes current generated in armature 5, via a short circuit path through diode 14 across field coil 6, just after cutoff of thyristors 10. Thus, rotating energy of motor 11 is consumed by resistance of armature 5 and field coil 6, and the motor 11 is dynamically braked and is rapidly stopped. Thyristor 13 does not receive a braking signal G13 during the braking period, but the current generated during motor rotation holds the thyristor conductive until it disappears. A diode 12 passes this current through field coil 6, and prevents this current from passing through only thyristor 13 and thus directly short-circuited.
FIGS. 5A and 5B show an embodiment of a control circuit embodying the gate circuits which were omitted in FIG. 3. Motor 11 is used for driving a sewing machine having a speed control and a brake. Thyristors 10 are fired by a phototransistor PTr1 receiving light from light-emitting diode LED1, which is driven by an output of operational amplifier OP4, further described below. In this control circuit, R1, R2 and R3 are resistors. D1 is a diode. ZD1 is a Zener diode for compensating voltage variations of power source 1. T is a stepdown transformer. Diodes D2 constitute a full wave rectifier. D3 is a diode. C1 is a voltage smoothing capacitor. R4 is a resistor. ZD2 is a Zener diode compensating for voltage variations of the power source 1 to provide a constant voltage between connections A and B. OP1 is an operational amplifier having an inverting input receiving a constant voltage divided by resistors R5 and R6 between connections A and B, and having a non-inverting input receiving a full wave rectified voltage from diodes D2, after division by resistors R7 and R8. The value of resistor 8 is set much larger than those of resistors R5, R6, and R7 which are of like value. Values of resistors R5 -R8 are so set that the power source period 0-π may be bigger than the (+) and (-) sides of operational amplifier OP1. ZD3 is a Zener diode for protecting operational amplifier OP1. OP2 is an operational amplifier having a non-inverting input receiving a divided voltage between the connections A and B from a potentiometer R9 adjusted by a machine control (not shown). Potentiometer R9 has a wiper CONT usually returned to its lowest end. As the wiper is moved up, the rotation speed of the motor is increased, and the motor is stopped when the contact is moved all the way down. The output side operational amplifier OP1 is connected to connection A via resistor R10, and is connected to connection B via a small resistor R11 and a capacitor C2. The output of operational amplifier OP2 is connected to its inverting and is also connected to a point between the resistor R11 and the capacitor C2, via resistor R12 and diode D7. These operational amplifiers OP1 and OP2 and capacitor C2 operate signals J1 -J4 (shown in FIG. 6) at the output of operational amplifier OP1. FIG. 6 indicates the speed of motor 11 in dependence upon position of the wiper CONT. The output of operational amplifier OP1 is connected to the non-inverting input of operational amplifier OP4. As is shown in FIG. 6, operational amplifier OP1 provides an output signal at time t1 just after power source 1 starts a cycle at time O. Capacitor C2 then starts to charge via resistor R10, and cuts off the output of operational amplifier OP1 at a point t2, just before the source begins a new cycle. Capacitor C2 then rapidly discharges via the resistor R11 and the operational amplifier OP1. Operational amplifier OP2 has an increasing voltage at its non-inverting input as wiper CONT shifts upwardly to thereby increase its output and thereby increase the charging speed of the capacitor C2. As a result, the charging potential is increased to enlarge the triangular signal from voltage J1 to voltage J4. Voltage J1 corresponds to the lowest position of the wiper CONT sewing basting stitches slowly. OP3 is an operational amplifier having an inverted input which receives a voltage divided by resistors R13 and R14 between the connections A and B, having a non-inverting input connected to the wiper CONT, and having an output terminal providing a low-level signal when wiper CONT is at its lowest position and providing a high-level signal when the wiper is at positions other than this lowest position. Thus, the operational amplifier OP3 checks the moving and stopping conditions of motor 11. An output signal C from operational amplifier OP4 lights a light-emitting diode LED via resistor R15 and operates the phototransistor PTr1 to control the phase of thyristors 10 in dependence upon the lighting phase of the phototransistor.
The circuit in FIG. 7 is used in association with the circuit in FIGS. 5A and 5B and includes a phototransistor PTr2. Phototransistor PTr2 is operated by a rotation speed detector on the main shaft of the sewing machine, and receives light pulses with a period which is proportional to the rotation speed of the sewing machine.
In FIG. 8, reference numeral 15 is an upper shaft of the sewing machine and 16 is a photointerrupter fixed to the sewing machine, including the phototransistor PTr2. A numeral 17 is a screen disc having a plurality of slits 18, and the phototransistor PTr2 is illuminated each time one of the slits 18 is opposite thereto.
In FIG. 7, the reference letter A indicates a connection to connection A in FIG. 5A and 5B. OP6 is an operational amplifier. R16, R17, and R18 are resistors and a terminal B is connected to terminal B in FIG. 5, constituting a Schmitt circuit together with phototransistor PTr2. As shown in FIG. 9, the emitter voltage D of phototransistor PTr2 has a substantially rectangular waveform relating to the width of the slits 18 in screen disc 17. Advancing to the right side, the width and the period of voltage D become smaller, because wiper CONT in FIG. 5 is progressively moved upwardly and the speed of motor 11 increases. The voltage E across capacitor C3 will be respectively the differential values of the rise and fall of voltage D. OP7 is an operational amplifier. R19 and R20 are resistors, C4 is a capacitor, which together with capacitor C3 form a frequency-voltage converter. The voltage E is integrated as is shown in FIG. 9 to form a speed signal which is proportional to motor speed. Each example of those values is shown as F1 to F4 in FIG. 6. Terminal F in FIG. 7 is connected to a terminal F in FIGS. 5A and 5B.
A reference numeral 19 in FIG. 8 is a part of photointerrupter 16 which includes phototransistor PTr3 in FIGS. 5A and 5B. Numeral 20 is a screen element having a large diameter through an angle of 180° and having a small diameter through an angle 180°. FIG. 10 shows the operation of the phototransistor PTr3 versus the rotation angle of the upper shaft 15 of the sewing machine which latter is represented on a horizontal axis. As shown in FIG. 10, phototransistor PTr3 becomes conductive just before the upper shaft 15 completes a half rotation to bring the needle to its upper dead point, and becomes nonconductive just before the upper shaft completes another half rotation to bring the needle to its lower dead point. Referring to FIGS. 5A and 5B, the collector of phototransistor PTr3 is connected to point A, and its emitter is connected through the inverter IN1 to an input of an exclusive OR circuit ExOR and is also connected to another input of the ExOR through a delay circuit composed of the resistor R21 and the capacitor C5. R22 is a resistor. The output of ExOR provides rising and falling pulses of a width equal to the time delay of the delay circuit, as shown in FIG. 10, at the upper and lower dead points of the needle. Such pulses are applied to AND circuits AND1 and AND2 for stopping the needle at the lower and upper dead points. The output of inverter IN1 is applied as a second input to the AND circuit AND1 and is also applied to another input of AND circuit AND2 through an inverter IN2. SWB is a switch for producing basting stitches. If switch SWB is closed, the sewing machine is driven at a low speed and it is possible to stitch intermittently by stopping the needle at the upper dead point on each of the sewing machine. When switch SWB is opened, one of the inputs of NAND circuits NA1 and NA2 becomes logically high, and becomes logically low when switch SWB is closed. SWC is a switch for stopping the needle. When switch SWC is opened, the needle is stopped at its lower dead point and when switch SWC is closed, the needle is stopped at its upper dead point. When switch SWC is opened, the other input of NAND circuit NA1 becomes logically high, and when the switch is closed that input becomes logically low. R23 are pulled-up resistors attached to point A. The output of NAND circuit NA1 is applied as a third input to AND circuit AND2 and is also applied as a third input to AND circuit AND1 via an inverter IN3. Basting stitch switch SWB is connected to the base of the transistor Tr1 via inverter IN4 and resistor R24. The collector of transistor Tr2 is connected to the connection between resistor R12 and diode D7 in order to ground the output of operational amplifier OP2 when switch SWB is closed. In this case, resistors R10 and R11 are so chosen that the output of the operational amplifier OP1 may form signal J1 in FIG. 6. Output Y of operational amplifier OP3 is connected to one input of AND circuit AND3 for detecting the rising of a signal Y on initial operation of the controller, and is applied to the other input of AND circuit AND3 through a delay circuit including inverter IN5, resistor R25 and capacitor C6. The output of the delay circuit is connected to set terminal S of flip-flop circuit FF. Signal Y is applied to the input of NAND circuit NA2 together with the signal from basting stitch switch SWB. The output of the NAND circuit NA2 is a first input to AND circuit AND4, for detecting when the motor is to stop. The output of AND circuits AND1, AND2 is applied to a second input of AND circuit AND4 via OR circuit OR. OP5 is an operational amplifier for detecting a low motor speed prior to stopping. Operational amplifier OP5 has a non-inverting input connected to the center point of a voltage divider formed by resistors R26 and R27, and has an inverting input connected to terminal F in FIG. 7, which carries the speed signal F graphed in FIG. 9. The output of the operational amplifier OP5 becomes logically high only when speed signal F becomes sufficiently low so as to cause the motor to stop, and this output is connected to a third input to AND circuit AND4. The output terminal of AND circuit AND4 is connected to a reset terminal R of flip-flop circuit FF. A complement side output Q of flip-flop circuit FF is connected to the inverting input of operational amplifier OP4 via diode D10, and is also connected to one input of NAND circuit NA3. A true output Q of flip-flop circuit FF is connected to the other input of NAND circuit NA3 via a delay circuit composed of a resistor R28 and a capacitor C7. The output of NAND circuit NA3 issues a base signal to a control transistor Tr2 via a resistor R29. Transistor Tr2 has its emitter connected to point A, and has its collector connected to a junction between resistor R15 and light-emitting diode LED1 via a resistor R30 and light-emitting diode LED2. Thus, transistor Tr2 is operated to light light-emitting diodes LED1 and LED2 concurrently. Light-emitting diode LED2 illuminates the phototransistor PTr4, delivering a gate trigger signal to braking thyristor 13. The collector of phototransistor PTr4 is connected to point X. R31 and R32 designate resistors.
The operation of the combined control circuits shown in FIGS. 5A, 5B and 7 will now be described. When basting stitch switch SWB is opened, the needle stop switch SWC is opened (causing the needle stop position to be the lower dead point), wiper CONT of the control (not shown) is positioned at its lowest position, and when AC source 1 is turned on, the operational amplifiers OP2 and OP3 have no output and the operational amplifier OP1 generates the low speed signal J1 shown in FIG. 6. But since output Y of operational amplifier OP3 is logically low, flip-flop circuit FF receives a low set signal at input S and maintains the reset condition which it assumes at power turn on. If AC source 1 is turned on when wiper CONT is positioned at an upper position, output Y of operational amplifier OP3 becomes logically high. But, capacitor C6 and inverter IN5 are logically low, AND circuit AND3 has logically low output and flip-flop circuit FF remains reset. Thus, flip-flop FF is maintained in a reset condition at the time of turnon of power source 1, even if basting stitch switch SWB and needle stop switch SWC are operated at such time. Hence, complement side output Q is logically high and the inverting input of operational amplifier OP4 is brought high, its output is brought low, and therefore, said LED1 is kept off, PTr1 is not illuminated, and motor 11 remains stopped. When wiper CONT is moved upward in FIGS. 5A, 5B (in case the wiper is at an upper position at the time of power turnon, wiper CONT must be once returned to its lowest position before it is moved up again), voltage at point Y is raised to a logically high level and flip-flop FF receives a signal at set input S for that time required by capacitor C6 to charge via resistor R25 during the rise of the signal Y on the transverse time axis shown in FIG. 11. When the signal Y is logically high NAND circuit NA2 has a logically low output and the signal at reset terminal R of flip-flop FF is also logically low, causing flip-flop circuit FF to remain set to generate speed signal F via operational amplifier OP4. F1 in FIG. 6 shows that when the motor is operating at a low speed, firing of thyristors 10 takes place at the time when signal J1 crosses speed signal F1. The phase range shown by oblique lines (in which signal J1 exceeds signal F1) varies as speed signal F varies. In this range, the operational amplifier OP4 has a logically high level output, and light-emitting diode LED1 thus lights in this range. Hence, the firing phase of thyristors 10 feeds back the motor speed. The respective crossing points of speed signals F1 -F4 and their respective speed designating signals J1 -J4 show the respective ignition phases of thyristors 10 in dependence upon the speed settings determined by movement of wiper CONT for speed control of the motor. Phototransistor PTr3 is at off and does not effect AND circuit AND2 since switches SWB, SWC are opened, and AND circuit AND1 issues a pulse after phototransistor PTr, is off. When the controller is released the signal at point Y becomes logically low. Since the operational amplifier OP5 has a high level output due to rotation of motor 11, the reset input R of flip-flop circuit FF becomes logically high and the flip-flop is reset at the initial conduction of phototransistor PTr3 after the signal at point Y was low, as is shown in FIG. 11. Thus, the output of operational amplifier OP4 becomes logically low, and light-emitting diode LED1 is not energized thereby. However, output Q of flip-flop circuit FF is high, and NAND circuit NA3 issues a low pulse having a width equal to the changing time of capacitor C7 and lights light-emitting diodes LED1 and LED2 during said pulse. This period has a large width exceeding the period π of power source 1, and light-emitting diode LED1 instantly short-circuits control thyristors 10 at time t5 in FIG. 4, and light-emitting diode LED2 then fires braking thyristor 13 and the field coil 6 is energized while armature 5 is not. The ignition signals for thyristors 10 and 13 instantly disappear, but armature 5 starts to generate electric power as it begins to rotate. As a result, a circulating electric current flows through braking diode 14, field coil 6 and thyristor 13 and the motor is dynamically braked. Motor 11 is stopped after a point which is just before the lower dead point of the needle has been detected (as shown in FIG. 11 for example, after rotation of around 30°), and the needle of the sewing machine is then set so as to just reach the lower dead point. When the needle stop switch SWC is closed, AND circuit AND2 is turned on and reset input R of flip-flop circuit FF is pulsed based on the rising current through phototransistor PTr3 in FIG. 11, and the needle thus stops similarly, but at the upper dead point. When basting stitch switch SWB is closed here, independently of operation of switch SWC, and wiper CONT is moved up, flip-flop circuit FF is set by the rising signal Y (shown in FIG. 12) and motor 11 is driven. Since transistor Tr1 now grounds the output of operational amplifier OP2, motor 11 is driven at a low speed. When the sewing machine is driven for one rotation and current through phototransistor PT3 is detected indicating a point just before the upper dead point of the needle, flip-flop circuit FF is reset and the sewing machine is braked as mentioned above and is stopped at the upper dead point of the needle. Subsequently, the machine is rotated once again and is subsequently stopped after one rotation to cause an intermittent stitching operation, such as basting, to be carried out. In FIG. 12, the operation Y of the controller is logically low after the stop signal NA3 is produced. If the controller is released before the stop signal is produced, braking is not influenced.
FIG. 13 shows another embodiment of the invention, in which elements which are same as those used in FIGS. 3 and 5A, 5B have the same reference numerals. 11a is a braking thyristor connected in series with a braking diode D10 and in parallel with the armature 5. G13a shows its gate terminal. R33 and R34 are resistors, and C8 is a capacitor. ZD4 is a diode for operating phototransistor PTt4. A junction between thyristor 13a and diode D10 is connected to AC source 1 via a diode D11 to supply an energized braking current to field coil 6 through thyristor 13a. Diode D10 prevents a current from being supplied to the armature 5 through diode D11.
The main difference between the embodiment of FIG. 3 and the embodiment of FIG. 14 is that braking thyristor 13a is ignited by a gate pulse G13a which has a width much narrower than the braking period, and is self-extinguished at time t8 when rotation N of motor 11a is stopped. If motor speed follows the dotted line in dependence upon motor load, the conductive period of the thyristor 13a will be postponed (as shown by the dotted line) until time t'8. This will now be explained with reference to FIG. 13. After thyristors 10 are ignited by the ignition signals G10 and motor 11a is rotated, if thyristors 10 are extinguished and thyristor 13a is fired at time t2 in order to brake motor 11a, the field coil 6 will be energized through diode D11 and thyristor 13a, and armature 5 will generate electric power on the basis of motor rotation at time t2. As a result, current circulating through diode D10 and thyristor 13a instantly stops the motor 11a. In this instance, the ignition signal G13a of thyristor 13 a is logically low, but thyristor 13a is kept conductive by the circulating current until power generation stops.
FIG. 15 shows that cutoff of thyristors 10 need not be coincident with firing of thyristor 13a, as is the case in FIG. 14. If the ignition signal G13a comes slightly earlier than the disappearance of the ignition signal G10, motor braking is initiated on the basis of the ignition signal G13a. In this case, current flowing through the thyristors 10 temporarily energizes field coil 6 through thyristor 13a, instead of through armature 5.
In FIG. 13, the braking current flowing through diode D11 is half-wave rectified. An additional diode D11 can be connected as shown to produce a full-wave rectified current so as to increase the braking effect.
In accordance with the invention as shown particularly in FIGS. 5A, 5B and 6, the signals J1 -J4 at the non-inverting input of operational amplifier OP4, (which is controlled by operation of the wiper CONT) start from zero potential at a zero crossover of power source 1, and end near the next crossover as substantially straight lines with different slopes. On the other hand, signals F1 -F4 at the inverting input of operational amplifier OP4 rise in proportion to motor speed. The crossing points of the two inputs of OP4 can control the ignition angle of the thyristors 10 in a broad range. In the case of this voltage comparison, only a voltage proportional to motor speed is used while voltage of the power source is not. Therefore, speed response is good. Especially during low speed rotation of motor 11, the inclination of signal J1 is small, causing the ignition phase of thyristors 10 to be broadly varied by any change in motor speed due to the variation of machine load. Thus, the invention provides an excellent control for motor speed and braking.
It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of sewing machine motor control systems differing from the types described above.
While the invention has been illustrated and described as embodied in a control system, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.
Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.